Numerous harmful microorganisms can be found in and on various food products. If untreated, these microorganisms can cause food to spoil prematurely and/or can cause sickness and even death. To increase the shelf life of food and decrease risks to consumers, many food products are pasteurized and/or sterilized (“Heat Sterilization”). In some instances, Heat Sterilization extends the shelf life of food products such that even otherwise perishable food products can be safely stored at room temperature for long periods of time (“shelf-stable”).
Heat Sterilization involves heating food products to a sufficient temperature (typically 80° C. to 100° C. for pasteurization and 100° C. to 140° C. for sterilization) and maintaining the food products at that temperature for a sufficient amount of time to kill harmful microorganisms. The time versus temperature relationship is based on a logarithmic scale such that a small increase in sterilization temperature has a significant impact on sterilization time. For instance, sterilizing a food product at 100° C. can take up to 200 hours while sterilizing the same food product at 115° C. or 122° C. may only require 6 hours or 6 minutes, respectively. In addition to the obvious benefits, minimizing sterilization time maximizes taste and nutrient retention while reducing the risk of physical damage to the food product.
Traditionally, Heat Sterilization was accomplished with heated air (“Dry Heat Sterilization”) and/or with heated vapors or fluids (“Wet Heat Sterilization”) using conventional heating methods of conduction and/or convection (“Surface Heat Sterilization”). More recently, volumetric heating (i.e. heating with energy waves such as microwaves or radio waves) has been used to sterilize food products (“Volumetric Heat Sterilization”). Each method has its advantages and disadvantages.
Boiling is a simple example of Wet Heat Sterilization. Unfortunately, boiling has various limitations. For instance, the boiling process prevents the Wet Heat Sterilization process from exceeding the boiling point of the boiling fluid (i.e. 100° C. for water). Because the sterilization temperature is so low, the sterilization time is very long. Additionally, to protect sterilized food from new microorganisms, Heat Sterilization is often accomplished while the food product is enclosed in an air-tight package. During the boiling process, the packaging and the air in the packaging heat up and expand. During the cooling process, the packaging cools faster than the air inside the packaging. Consequently, as the packaging shrinks, the internal pressure increases even more, sometimes causing the packaging to burst.
Retort Sterilization, which involves heat and pressurization, solves some of the problems associated with boiling. Because Retort Sterilization involves pressure, the boiling temperature of the fluid is increased. This increases the sterilization temperature, thereby decreasing the required sterilization time. Additionally, the ability to control the pressure in the system can be utilized to prevent the packaging from bursting. U.S. Publication Number 2013/0071546, the entire disclosure of which is incorporated herein by reference, describes Retort Sterilization and other sterilization processes in further detail.
Although Retort Sterilization is a superior process compared with other conventional sterilization processes, the Retort Sterilization process is still a Surface Heat Sterilization process that can take several hours to complete. Recently, work has been done to utilize Volumetric Heating to sterilize food products. Volumetric Heating is capable of simultaneously heating the entire volume of a food product. U.S. Pat. No. 7,119,313, U.S. Pat. No. 8,586,899, and U.S. Publication Number 2014/0083820, the entire disclosures of which are incorporated herein by reference, describe Volumetric Heating and other sterilization processes in more detail.
Volumetric Heating drastically reduces the amount of time necessary to sterilize products, in some cases decreasing the energy required to sterilize the product and/or increasing the shelf life, taste, and/or nutritional value of the sterilized food product. Unfortunately, Volumetric Heating is expensive and complex. Additionally, various other problems have prevented Volumetric Heating from becoming commercially successful.
Volumetric Heating can be unpredictable. For instance, it is difficult or impossible to predict how energy waves will flow through a particular food product, making it difficult or impossible to predict how thoroughly a food product will be heated. To reduce some of this uncertainty, systems of the prior art have been forced to use microwave emitters that emit microwaves at 915 megahertz (“MHz”) rather than the more readily available microwave emitters that emit microwaves at 2450 MHz because microwaves at 915 MHz generally heat more thoroughly than do microwaves at 2450 MHz. Furthermore, systems of the prior art have been forced to use single mode microwave systems rather than multimode microwave systems. Furthermore still, some systems of the prior art require the use of water and/or other fluids to reduce uncertainty by decreasing a phenomenon known as “edge heating.” Unfortunately, while solving some problems, these solutions create additional problems. For instance, U.S. Pat. No. 7,119,313 teaches filling a cavity of a container with a fluid, placing a food product in the cavity such that it is submerged in the fluid, pressurizing the cavity, and propagating microwaves at 915 MHz into the cavity of the container so as to heat the food product and the liquid (“Volumetric Retort Sterilization”). Unfortunately, the temperature of the fluid is elevated to such that its ability to reduce edge heating is minimized. Additionally, as the temperature of a liquid such as water approaches the temperature of the liquid's latent heat of evaporation, Volumetric Heating becomes less effective and even less predictable. Consequently, it would be beneficial to have a more predictable system and method of utilizing Volumetric Heating to pasteurize and/or sterilize food products.